This report covers a theoretical and experimental investigation of methods of improving the conversion efficiency of S-band Klystron-TWT Hybrid approximately 1 MW of peak power. The 'short stack' intense interaction cloverleaf structure was further developed as the output section of an S-band Twystron. Additional mode loading cavities were added. The coupling slot lengths were lengthened. The two high level bunching cavities in the Klystron section were modified. The gun used in prior work was modified to improve the beam-circuit interaction. Computer-aided calculations predicted 37% minimum efficiency over a 10% bandwidth with a midband efficiency of 51%. The additional mode loading rendered the tube absolutely stable. The elimination of the higher order mode in the two klystron cavities provided a significant improvement in the gain response. The increase is slot lengths seemed to reduce the magnitude of the efficiency dip. The gun modification and the increase in slot length worsened the beam-circuit interaction as judged from measured efficiencies of only 3/5 of those predicted. Performance at the design voltage and current is representative of performance in all regions tested. The swept response showed a midband efficiency of 27% with a 1.5 dB bandwidth of 325 MHz. (Author)

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